The autotrophic bacterium, Nitrosomonas europaea, derives energy from the oxidation of ammonia to nitrite. Oxidation of the intermediate, hydroxylamine, is catalyzed by hydroxylamine oxidoreductase. Electrons pass from this enzyme to the tetraheme cytochrome c-554. This cytochrome is thought to direct some electrons to an electron transfer chain leading to ATP-synthesis. The four hemes in cytochrome c-554 are magnetically coupled, giving rise to a single unusual EPR feature. The goal of NMR studies is to elucidate details of this magnetic coupling. The studies are aimed at determining (a) the axial ligands of the hemes, (b) the nature of amino acid groups that interact with the hemes and (c) possible interactions between hemes. NOESY spectra with different mixing times (30, 50, 70 and 80 msec), one-dimensional NOE spectra and a COSY spectrum have been recorded. We have concluded that unambiguous assignments are not possible for the oxidized protein with unlabeled hemes because of difficulties in distinguishing intra- and interheme connectivities.